Any suggestions for longer names that can go by [name_f]Katie[/name_f] that aren’t Catherine/Katherine etc?
Assuming they can be in the [name_f]Katherine[/name_f] family…
[name_f]Katelyn[/name_f] / [name_f]Caitlin[/name_f] / [name_f]Kaitlin[/name_f]
[name_f]Katia[/name_f] / [name_f]Katja[/name_f]
[name_f]Katrina[/name_f]
[name_f]Hecate[/name_f]
[name_f]Kathleen[/name_f]
[name_f]Katara[/name_f]
[name_f]Katarin[/name_f]
[name_u]Cataleya[/name_u]
[name_u]Cato[/name_u]
i love previous suggestions of Katarin and Katja (+ variants), and i second Kaitlin & Hecate, too.
Katarina/Katerina
Katalina
Katiana
Katica
Katinka
Katrine
Kaitria
it’s not longer, but just Kate would also work.
Maybe a stretch, but [name_u]Cadence[/name_u] or [name_u]Kadence[/name_u] - since some people say [name_f]Kady[/name_f] and [name_f]Katie[/name_f] in a similar way
[name_u]Katriel[/name_u]
[name_f]Cataline[/name_f]
[name_f]Katharina[/name_f]
[name_f]Katana[/name_f]
[name_f]Katarzyna[/name_f]
[name_f]Katla[/name_f]
[name_f]Katka[/name_f]
[name_f]Katonya[/name_f]
[name_f]Katuska[/name_f]
[name_f]Katusha[/name_f]
[name_f]Katushka[/name_f]
[name_f]Acatia[/name_f]
[name_u]Catesby[/name_u]
[name_f]Catrika[/name_f]
[name_f]Catrinel[/name_f]
Eikaterina
[name_f]Catrona[/name_f]
[name_f]Catrelle[/name_f]
[name_f]Acadia[/name_f] / [name_f]Arcadia[/name_f]
[name_f]Cadena[/name_f]
[name_f]Cadenza[/name_f]
[name_f]Cadia[/name_f]
[name_f]Leocadia[/name_f]
[name_f]Elocadia[/name_f]
[name_f]Arkadina[/name_f]
[name_f]Kadida[/name_f]
[name_f]Kadiatou[/name_f]
Kadria
[name_f]Lacadia[/name_f]
I mean, I know a [name_f]Katie[/name_f] who is just [name_f]Katie[/name_f]. I think it works perfectly well on its own if you can’t fine a longer version you love as much. 
[name_f]Katarina[/name_f]
[name_f]Katia[/name_f]
[name_m]Kaiden[/name_m]